For many years, various brightfield, darkfield, and e-beam scanning methodologies have been used to inspect surfaces. These scanning technologies make use of radiation scattered, diffracted, and/or reflected by a surface to characterize and examine features of the surface. In particular, such scanning technologies are used to examine surfaces to determine the presence and location of defects in an inspected surface. The details of these and other related scanning and inspection technologies are well known to those having ordinary skill in the art.
Typically, such devices examine a surface by projecting a light beam onto the surface and then detecting resulting pattern of light received from the surface. Also typically, light beams are projected onto the surface over a single frequency (e.g., as typified by laser devices) or over a bandwidth of frequencies (such as can be obtained using appropriately filtered arc lamps).
In an effort to identify defects of ever decreasing sizes, conventional tools typically rely on illumination sources having shorter and shorter wavelengths. As is well-known to those having ordinary skill in the art, illumination of a surface with shorter wavelengths of light can be used to obtain greater resolution in images of the surface. Previously, greater resolution has been thought to be the key to improving defect detection of smaller defects. The inventor has discovered that greater resolution is only part of the defect identification story.
A common implementation of a conventional inspection tool is shown by the simplified schematic representation of a bright field inspection tool 100 shown in FIG. 1. An object 102 (commonly, a wafer) is secured to a movable stage 101 where it is illuminated by a light beam 103 produced by a single light source 104. Commonly, the light source 104 comprises a single arc lamp selectively filtered to produce a defined bandwidth of frequencies. In some alternatives, the light source 104 comprises a laser tuned to produce a single frequency coherent beam of light centered at some peak wavelength. Commonly, the light beam 103 is directed onto a partially transmissive surface 105 that reflects a portion of the light beam 103 through an objective lens system 106 which directs the beam 103 on the surface 102. Typically, different portions of the object 102 are successively inspected by scanning the substrate. Commonly, the stage mounted object 102 is scanned by moving the stage 101 as directed by a scanning control element 110. As the surface 102 is scanned, light reflected, scattered, diffracted, or otherwise received from the surface 102 passes through the objective 106 and passes back through the partially transmissive surface 105 into an optical system 107 that optimizes the light for detection with a detector system 108. Commonly, such optimization includes magnification, focusing, as well as other optical processing. The detector 108 receives the light and generates an associated electrical signal which is received by image processing circuitry 109. The processing circuitry conducts defect analysis to locate defects in the object 102.
Conventional approaches to detecting defects and process variations in layers of a semiconductor substrate have operated under the assumption that greater resolution in an inspection tool translates into greater sensitivity to the presence of defects. The resolving power of optical inspection tools can be characterized by the “point-spread-function” (PSF) of the tool. The PSF is affected by a number of factors including, but not limited to the optical quality of the lenses (or other optical elements) used in the focusing elements, the wavelength of light, numerical aperture (NA) of the objective lens system, pupil shape, as well as other factors. In general, the resolution of a system is related to the wavelength (λ) of the exposure source divided by the numerical aperture (NA) of the objective lens system. Therefore, shorter wavelengths can be used to produce greater resolution.
Thus, the prior art the emphasis has generally been on generating inspection tools that use shorter wavelength inspection sources to obtain better resolution, and presumably better sensitivity to defects. Light sources operating at visible wavelengths and near ultraviolet wavelength (e.g., from about 400 nm to about 300 nm) have long been used as illumination sources. For example, mercury (Hg) or mercury xenon (Hg Xe) arc lamps have been used as illumination sources in the near ultra-violet (UV) and visible ranges. But, in the continuing drive for greater resolution, shorter wavelength illumination is desirable. Because the power of Hg and HgXe sources drops off rather dramatically, below 300 nanometers (nm) they are not excellent sources of deep UV (e.g., wavelengths below about 300 nm) illumination. Deep UV (DUV) lasers are a commonly used illumination source in the DUV range. Due to the rather high cost of such sources (especially, the laser sources), no known inspection tools or methods have implemented two (or more) sources to inspect surfaces. In fact, until now there was no reason to attempt to do so. The conventional approach has generally been to devise the shortest wavelength system and implement such a system to obtain the best possible resolution.
Although existing inspection machines and processes accomplish their designed purposes reasonably well, they have some limitations. There is a need for greater sensitivity than is currently provided by existing machines and processes. For these and other reasons, improved surface inspection tools and methodologies are needed.